Hi,  tablets are not good at your age. Better to choose a healthy way to gain weight rather than eating tablets . Side effect weight gain tablets differs from person to person. We treat the disease by treating the cause and the result will increased weight. There are many reason of being underweight. 1. Endocrine disturbances 2. Genetic 3. Vitamins and minerals deficiency 4. Psychological problem 5.IBS 6. Eating disorder 7. Liver dysfunction And many more.. So my opinion is to contact any near by dietician or if you want you can consult me directly through this link